Measuring instrument.



E. THOMSON.

MEASURING INSTRUMENT. APPLIOATION FILED MAY 11, 1908.

Patented June 15, 1909.

INEE' A LMVENTUE; ELIHU IHDA/ISDN.

I/YITNEESES" 1HE :wRms PETERS co.. wAsHmcran, u4 c.

UNTTEB STATES ELIHU THOMSON, OF SWAMPSCOTT, MASSACHUSETTS, ASSIGNOR TOGENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

MEASURNG INSTRUMENT.

To all whom it may concern.'

Specification o Letters Patent.

Application filed May 11, 1908.

j to be compared very exactly regardless of Be it known that l, ELIHUTHOMSON, a magnetic fields or of irregularities in the citizen of theUnited States, residing at Swampscott, county of Essex, State ofMassachusetts, have invented certain new and useful Improvements inMeasuring lnstruments, of which the following is a specification.

My invention relates to measuring instruments, particularly electricalmeasuring instruments of the hot wire type, and its object is to providea simple and reliable electrical measuring instrument unaffected bymagnetic fields or by changes in the wave form of a current and by meansof which an alternatingv current can be measured accurately or can becompared very exactly with a direct current, which in turn can bereferred directly to a primary standard and measured more accuratelythan an alternating current.

In carrying out my invention, a tone varying with the quantity ofcurrent to be measured is produced, and the extent of the variation inthe tone is measured in any suitable way, preferably by an adjustingmeans which enables the tone to be varied independently of the currentuntil brought into unison with some standard tone, the extent of thisadjustment being a measure of the current flowing through the device.Any suitable device may be used to produce the tone, the preferable formbeing a pipe, similar to an organ pipe, which is blown by compressed airor other suitable fluid and in which the tone is varied in accordancewith the variations in said current by varying the density of the air inthe pipe in any suitable way, preferably by means of a heat'- ing coilresponsive to the current to be measured and mounted in the pipe to heatthe air. The variation in the tone of the pi e is preferably measured bymeans of an adpjustable stop in the pipe, which enables the tone of thepipe to be changed independently ofthe current and to be compared withsome standard ifdesired. lll/*here two different currentsare to becompared a device constructed as Vabove described is used for eachcurrent, and if the two devices normally sound in unison the amount ofadjustment required to make them sound in unison while the two currentsare flowing measures the two currents and enables them wave form ofeither current.

My invention will best be vunderstood in connection with theaccompanying drawings, which are merely illustrative of one form inwhich it may be embodied, and in which- Figure l is a plan view partlyin section of a device embodying my invention; Fig. 2 is a longitudinalsection on the line 2--2 of the device shown in Fig. 1'; and Fig. 3 is adiagrammatic view showing the instrument as used for comparing twodifferent currents.

ln the specific form of instrument shown in the drawing, the base l ofany suitable material carries a box 2 having a removable cover andcarrying on each side a pair of terminals 3 and 4 by means of' whichdifferent currents can be introduced into the instrument. The meansshown for producing a tone which willvary with the current to bemeasured, comprises a source of supply 5 for compressed air or othersuitable fluid which blows pipes 6 and 7 constructed in the same manneras an organ pipe. The velocity of sound through a column of air in apipe of this type is very nearly equal to the square root of the airpressure divided by its density, and when the value of Athis 'ratio ismaintained constant, the pitch of' the pipe is also constant. Barometricchanges of pressure do not affect the pitch because the density ol' theair will vary directly with the pressure, but if the temperature oi' theair is changed the density at any given pressure will vary inverselywith the temperature. if the temperature of the air rises thedensitywill fall, the ratio of the pressure and density is no longerconstant, the velocity of the sound waves therefore increases and thepitch of the pipe is correspondingly raised. rPhe changes of pitchproduced in this way are taken to vary as the square roots of' theabsolute temperatures, that is 2730 plus the temperature above OO C. f

ln order to maire the pitch of the tone produced by the pipes 6 and`7dependentl on the current to be measured, the density of the air orother fluid in the pipes is caused to vary with the current by means ofthe heating coils 8 placed in the pipes and connected to the terminals 3and a. These heating coils develop heat varying with the current flowingthrough them and thereby heat the surrounding air in the pipe, changingthe density of the air and therefore changing the pitch of the soundemitted by the pipe.

The extent of variation in pitch produced by the current owing throughthe heating coil 8 may be measured in any suitable manner, but mostconveniently by providing some adjusting means which enables the pitchof the pipe to be altered. independently ofthe heating coil. rlheadjusting means shown in the drawing comprises a stop 9 for the pipeaduiustable to vary the length of the column of air in the pipe andcarried upon an adil usting screw l0 mounted in a boss on the box 2 andprovided with a micrometer head ll which cooperates with a scale 12 andypermits the position of the stop in the pipe to be determined veryaccurately. lf the pipe is blown and produces a tone of a certain pitch,the passage of a current through the heating coil will alter the pitchand the amount of adjustment of the stop 9 which is requiredto make thepitch when current is flowing the same as the pitch when no current isflowing is an accurate indication of the amount of current flowingthrough the coil.

The instrument is calibrated by adjusting the air pressure until thepipes produce a tone that can be clearly heard and the pipes are thenmade to sound in unison. A current of known value is then passed throughthe resistance in one of the pipes,heating the air and raising the pitchof that pipe. The plug?) at the top of the pipe is then lowered into thepipe until the notes of both pipes are again in unison and beats are nolonger heard when the position of the micrometer screw is noted. Otherpoints in the calibration are obtained in the same way and a tablegiving the settings of the micrometer screw for different currentsmaires it possible to measure any current within the compass of theinstrument.

As shown in Fig. 8, it is possible by means of this instrument tocompare a direct current with an alternating current, and in using theinstrument in this way the pipes are blown in unison without any currentpassing through the heating coils, after which the two currents to becompared are passed through the heating coils of the respective pipes,thereby changing the pitch of each pipe. The stops in the pipes areadjusted to bring the tones of the two pipes again into unison and thereadings are compared.

The length of the pipes and the pitch of the tone employed is immaterialbut if a pipe having a length of about 2st is used, the pitch of thenote will have afrequency of approximately 3000 periods per second. ifit 1s assumed that this frequency is obtained at 20o C. then thefrequency at 21o C. bears the same relation to 3000 that the absolutetemperature at 210 C. bears to the absolute temperature at 200 C., or inother words the frequency at 21 C. becomes 3005 periods per second and arise of lo C. sharpens the pitch in the ratio of 3000 to 3005. Thenumber of beats will be the difference between these two frequencies orlive per second.

My invention may be embodied in many other forms than that shown anddescribed, and l therefore do not wish to be restricted to the exactform shown but intend to cover by the appended claims all changes and.modifications within the spirit and scope of my invention.

What l claim as new and desire to secure by Letters Patent of the UnitedStates, is,-

l. in an electrical measuring instrument, the combination with means forproducing a tone, of means responsive to the current to be measured forvarying said tone with variations in said current, and means formeasuring the variations in said tone.

2. In an electrical measuring instrument, the combination with means forproducing a tone, of means res onsive to the current to be measured forren ering the pitch of said tone dependent on the current to bemeasured, and means for measuring the pitch of said tone.

3. ln an electrical measuring instrument, the combination with means forproducing two tones, of means dependent on the current to be measuredfor varying one of said tones, and means for varying one of said tonesindependently of said current.

4. An electrical measuring instrument comprising a current carryingconductor, an elastic medium responsive to changes of temperature ofsaid conductor, and means for indicating the changing vibration ratesunder such changes of temperature.

5. In `a measuring instrument, the combination with means for producinga tone, of means dependent on the quantity to be measured for varyingthe pitch of said tone, and adjusting means for varying the pitch ofsaid tone independently of said quantity.

6. ln a measuring instrument, the combination with means for producing atone of uniform pitch, of means for producing a second tone, meansdependent on the quantity to be measured for varying the pitch of saidsecond tone, and adjusting means for varying the pitch of the secondtone independently of said quantity.

7. In a measuring instrument, the combination with means for producing atone dependent in pitch on the density of a fluid, of means forrendering the density of said fluid dependent on the quantity to bemeasured.

8. In an electrical measuring instrument, the combination with means forproducing a tone dependent in pitch on the densityy of a fluid, of aheating coil responsive to the current to be measured for varying thedensity of said iiuid.

9. In a measuring instrument, the combination with a pipe and means forsounding said pipe, of means for rendering the density of the air insaid pipe dependent on the quantity to be measured.

l0. n an electrical measuring instrument, the combination with a pipeand means for sounding said pipe, of a heating coil in said.

pipe responsive to the current to be measured.

1 1. ln a measuring instrument, the combination With a pipe and meansfor rendering the pitch of the tone of said pipe dependent on thequantity to be measured, of adjusting means for var fing the pitch ofsaid pipe independently of said quantity.

12. In a measuring instrument, the combination with a pipe and means forrendering the pitch of the tone of said pipe dependent on the quantityto be measured, of an adjustable stop in the end of said pipe forvarying the pitch of the tone independently of said quantity.

13. In an electrical measuring instrument,

the combination With a plurality of devices for producing tones, ofmeans dependent on the current in a circuit for varying the pitch of thetone of each of said devices, means for connecting each device to acircuit, and adjusting means for varying the tone of one of said devicesindependently of the current.

14. In an electrical measuring instrument, the combination with aplurality of pipes and means for rendering the tone of each pipedependent on the amount of current supplied to it, of means forconnecting each pipe to a circuit, and an adjusting device for varyingthe pitch of the tone of one of said pipes independently of the current.

In Witness whereof, I have hereunto set my hand this eighth day of May,1908.

ELIHU THOMSON.

Witnesses JOHN A. McMANUs, Jr., CHARLEs A. BARNARD.

